1. Technical Field of the Invention
This invention relates generally to the measurement of toner particle mass deposition, deposition rate, and charge-to-mass ratio in electrostatographic reproduction apparatus such as copiers and/or printers. More particularly, it relates to improvements in an apparatus for monitoring certain properties which characterize the electroscopic toner particles used in electrostatographic recording devices to develop electrostatic charge patterns and images.
2. Description of the Prior Art
Electrostatographic apparatus, such as electrophotographic copiers and printers which use a dry powder toner to develop latent images, usually require some type of monitoring and controlling device for maintaining consistency in the quality of the developed image. Toner concentration monitors are frequently used and a variety of systems are seen and described in the prior art. The need for such systems is especially important when the size of the toner particle is reduced to allow for higher resolution in the developed image.
In an article G. J. Sem, K. Tsurubayashi and K. Homma, J. Am. Ind. Hyg., Assoc. Vol. 38, p. 580 (1977), the use of a piezoelectric microbalance is disclosed as a respirable aerosol sensor. In that study, a 5 MHz AT cut quartz transducer was used to sense the presence of various types of industrial contaminants including CaCO.sub.3 particles, welding fumes, cotton mill dust, powdered metal, and tobacco smoke. The particle concentration investigated in that study was (0.05-5.5 mg/M.sup.3), which is far lower than that encountered in xerographic conditions. There was no discussion in that article of the sensitivity of the disclosed device at higher particle concentrations. In fact, their discussion of the limits of sensor loading (page 588) suggests that the device would not function in a xerographic development station. Also disclosed in their results was the fact that it only applies to 3.5 micron diameter particles. In contrast, a piezoelectric development sensor is designed to work with toner particles having diameters between approximately 1.0 and 15 microns.
G. Sauerbrey, Zeitschrift fur Physik, Vol. 155, 1959, p. 206 and G. Sauerbrey, Zeitschrift fur Physik, Vol. 178, 1964, p. 457 reported that for transducers comprising a free-standing quartz plate, the sensitivity of the transducer to the presence of particles increased towards the center of the transducer.
U.S. Pat. No. 5,006,897, issued in the name of Rimai et al on Apr. 9, 1991, is directed to an apparatus used to detect the mass deposition rate of toner particles during the xerographic development process. Specifically, the front surface of the piezoelectric element is placed in close proximity to the development station containing xerographic developer. The front surface of the piezoelectric element is biased so as to attract toner from the development station. In addition, an AC bias is superimposed onto the DC bias at the resonant frequency of the piezoelectric element. By measuring the shift in resonant frequency, the mass of the deposited toner and the mass deposition rate can be determined. After depositing the toner mass onto the front surface of the piezoelectric element, the bias voltage is reversed and the action of the development station removes the toner, thereby preparing the sensor for subsequent measurements.
In order for this sensor to function properly, it must be firmly clamped into a fixed position so as to preclude the occurrence of artifacts induced by spatial variations or vibrations of the sensor. As is well known, clamping a piezoelectric transducer significantly alters its response to either an applied electrical or mechanical signal. In addition, it is necessary to preclude the deposition of toner particles from the side of the transducer opposite the front surface of the transducer, i.e., the rear surface. This requirement further complicates the necessary clamping and clamping fixtures and can further affect the response of the transducer.
It is necessary to bias the front surface of the transducer in order to attract toner. The application of such a bias can result in a mechanical constriction of the transducer, thereby giving rise to so-called "piezoelectric stiffening". This will affect the response of the transducer to further mechanical stresses or loads. Finally, it is necessary to remove the toner particles from the transducer following a measurement. Because of the necessary interactions between the transducer and the development station in order to effect such cleaning, toner deposition outside of the developer nap must be avoided.
As mentioned above, for use as a xerographic development sensor, the front surface of the transducer must be biased in order to attract toner. This is in marked contrast to the aerosol sensors discussed above where random deposition of the particles on the sensor surface takes place. As a consequence, it would appear that reducing the area of metallization on the front surface of the xerographic development sensor would actually reduce the amount of toner deposited onto the transducer and less toner deposition would be expected to result in less variation of the shift in resonant frequency of the device and would suggest that, by reducing the metallization, the device should be less sensitive.